1. Field of the Invention
The present invention relates generally to devices and methods for defrosting of the cold sides of the refrigeration units. The present invention relates more specifically to the structure and composition of a device to rapidly and controllably defrost the evaporators and other under freezing sides of the refrigeration or heat pumping units using waste energy rather than added energy.
2. Description of the Related Art
The frost deposit on the evaporators represents a natural and unwanted phenomenon. In refrigeration and deep freezing units during the cooling cycle time the evaporator reaches lower temperatures than the freezing point of water, (0.degree. C.), so, the frost, that is deposed will lower the overall efficiency of the units due to the thermal resistance of the snow-like deposits. It is well known that the frost deposit cannot be avoided because the water vapors are always present in the air infiltrating the unit. Realistically it can be said that most of the time the evaporators are covered with some unwanted frost which directly increases the energy required to keep the temperature of the refrigeration unit within necessary limits.
There have been a number of attempts to create devices and methods for an intermittent defrost cycle. The most commonly used device is based on an electric resistance heater, which is placed nearby the evaporator. The heat produced is used to melt the frost and the resulting water is collected and guided out of the system. The undesirable characteristic of this method is the relatively long time necessary to defrost because of the poor heat transfer (free convection of the air) from the electrical resistor to the evaporator coils. The residual heat in the evaporator space after defrost will increase the input energy necessary to regain the temperature needed inside the refrigeration unit. Usually the signal for defrost cycle is received from a sensor mounted directly on the evaporator which can measure the temperature drop of the evaporator due to the frost deposit. The temperature drop is directly influenced by the thickness of the frost.
Other defrost devices replaced the well-known electrical resistor with an infrared heater. The resistor wire is encased in a quartz glass tube and its operating temperature is set at red-orange color. The infrared radiation is reflected by the walls of the evaporator space and when they reach the evaporator, the thermal effect will melt the frost. This kind of infrared generator is mostly used when the evaporator is placed horizontally and free air connection cannot be used properly. The disadvantage of this approach is due to poor absorption of the infrared radiation by the frost, which is white and therefore reflective. The time necessary for defrost will thus be increased and the residual heat within the evaporator space is increased.
Another method for defrosting is the "reversed cycle" technique. Mostly used for larger units, the idea is based on in reversing the circulation of the refrigerant fluid inside the refrigeration system. In this way the evaporator will become the condenser and will receive hot, pressurized refrigerant from the compressor. The heat dissipated by the pressurized refrigerant is used to melt the frost. The "reversed cycle" or defrost cycle is achieved by solenoid valves that reverse the usual circulation of the refrigerant The valves are activated by the defrost sensor, as it was previously mentioned. The disadvantages of such a system are obvious: more moving parts within pressurized pipes should decrease the reliability, and the thickness of the evaporator walls must be increased to comply with the pressurized side requirements. As a result the thermal resistance of the evaporator will increase and the thermal transfer between the refrigerant and the air will decrease; so, the coefficient of performance (C.O.P) of the system will be lower than that of a classic unit.
The above existing devices and methods for defrost were developed in order to increase the efficiency of the refrigeration units. Energy savings represent a stringent requirement for all fields; global warning and the related aspects represent a global concern. High-energy efficiency appliances are required to be developed as part of the energy saving effort. Refrigeration systems of all kind represent one of the most important energy consumers.